Field
This invention relates generally to a radiometric system for passively detecting and thermally imaging objects in a scene and, more particularly, to a radiometric system for passively detecting and thermally imaging objects in a scene, where the system employs a digital square-law quantizer and delta-sigma feedback integrated on a common chip using, for example, silicon-germanium (SiGe) and/or Si complementary metal oxide semiconductor (CMOS) fabrication technologies.
Discussion
Radiometric thermal imaging systems and cameras are known in the art that passively detect and process signals in certain frequency bands emitted from a particular scene. Some of these frequency bands, such as various RF bands in the 60-300 GHz frequency range, for example, 94 GHz (W-band), 140 GHz (D-band) and 220 GHz, are particularly useful because the emissions in those bands readily propagate through clouds, smoke, dust, fog, etc. Typically, the warmer or more emissive the object in the scene, the more energy the object emits at a particular frequency. The radiometric system will detect the RF energy and convert it to a representative temperature value so that the different objects in the scene can be separately imaged and identified. In order to be suitable and effective, these radiometric systems generally require detectors that have a high sensitivity, low drift and can handle large background temperatures.
A typical radiometric camera that detects and images radiation in the RF bands often includes a focal plane array (FPA) that converts the radiation into an electric signal, where a lens focuses the radiation onto the array. The FPA typically includes a configuration of a plurality of receivers positioned in a two-dimensional plane, where each of the receivers includes an antenna or signal horn having a pick-up probe at the front end that converts the radiation to an electrical signal that is amplified by a microwave monolithic integrated circuit (MMIC) low noise amplifier. A diode at the back end of the each receiver rectifies the amplified voltage signal to a DC signal, where the DC signal amplitude is representative of the power level of the received signal, which increases as the temperature of the object being imaged increases, and where power and temperature are proportional to each other. The DC voltage signal from each receiver is then digitized and converted to an image, where higher voltages are displayed as whiter areas in the image representing warmer objects with higher radiometric temperature.
One of the challenges associated with these types of radiometric cameras is providing a high enough thermal resolution between objects detected in the scene. For example, in a typical scene, many of the objects will be near room temperature, thus requiring significant temperature resolution to distinguish those objects from each other. The sensitivity of the radiometer system depends on the receive system noise temperature T, the captured radiometric bandwidth B and the integration time τ as follows where it is assumed that the detector does not contribute input-referred noise power, and that low frequency bias drift is not present.
      δ    ⁢                  ⁢    T    =                    T        Sys                              B          ⁢                                          ⁢          τ                      .  
It is desirable that the operation of the radiometric system be as immune from signal drift as possible as the temperature of the system itself changes. In order to overcome a significant portion of this analog drift, it is known in the art to provide digital radiometric systems where the received RF analog signal is converted to the digital domain as quickly as possible. However, there are advantages and disadvantages to providing different levels and degrees of analog circuitry and digital circuitry in these types of systems. For example, the antenna and amplifier capture signals over a wide range of frequencies B. Analog diodes can rectify an entire broad frequency range. However, these types of analog circuits that employ diodes and the like often times have unacceptable signal drift and noise, especially at lower frequencies. Further, diode detectors typically do not have enough nonlinearity and are inefficient. Also, known digital radiometric systems that may employ fast Fourier transform (FFT) or root mean square (RMS) operations that convert digital frequency signals to a representative temperature typically require significant calculation at high sampling and data rates.